1. Field of the Invention
The present invention relates generally to apparatuses detecting abnormality of internal combustions engines and particularly to those identifying abnormality of exhaust systems.
2. Description of the Background Art
For a vehicle's internal combustion engine an air intake system intakes air which is in turn mixed with a fuel provided from a fuel system and the air fuel mixture is combusted in a combustion chamber and an exhaust system exhausts gas. Accordingly the internal combustion engine's operation is controlled as based on detection signals issued from the air intake, fuel and exhaust systems, respectively.
For such an internal combustion engine the exhaust system is provided with an exhaust sensor outputting a signal which is used to provide a feedback learning value allowing an air fuel ratio (or air fuel mixture) to be controlled by feedback to be a theoretical air fuel ratio.
If the exhaust system or the like has trouble, however, the internal combustion engine's each system is unsatisfactorily controlled. Accordingly an internal combustion engine is equipped with an abnormality detection apparatus detecting abnormality of the internal combustion engine including an exhaust system. For example a technique employed to detect an internal combustion engine's abnormality, an exhaust system's gas leak and other similar abnormality is disclosed in the following documents:
Japanese Patent Laying-Open No. 10-47130 discloses an apparatus detecting abnormality of an internal combustion engine that accurately, separately identifies abnormality of an air intake system, that of an exhaust system and that of a fuel system. This apparatus, in an internal combustion engine including an exhaust system provided with an exhaust sensor outputting a signal used to provide a feedback learning value allowing an air fuel ratio to be controlled by feedback to be a theoretical air fuel ratio, includes an abnormality determiner. More specifically, a condition is established for diagnosis of abnormality of the internal combustion engine, a feedback learning value and a preset feedback learning value are compared, a feedback learning value varying with the internal combustion engine's condition of operation and preset feedback learning value are compared, and the internal combustion engine's abnormal portion is determined by the abnormality determiner. The internal combustion engine's abnormality portion can early be specified to prevent large amounts of exhaust gas released into the atmosphere and negatively affecting the environment.
Japanese Patent Laying-Open No. 2003-206805 discloses an engine's air fuel ratio control apparatus diagnosing with precision an exhaust gas leak introduced upstream of a catalyst. This apparatus includes air fuel ratio sensors arranged upstream and downstream, respectively, of the catalyst, a portion calculating a basic control constant for air fuel ratio feedback control, a portion determining whether it is a time at which a condition for the air fuel ratio feedback control is established, a portion driven by a decision that it is a time at which the condition is established to use the downstream air fuel ratio sensor's output to calculate a modified value for the basic control constant, a portion using the modified value to modify the basic control constant to calculate a control constant, a portion using the calculated control constant to control by feedback an air fuel ratio as based on the upstream air fuel ratio sensor's output, and a portion determining that exhaust gas is leaking when a difference between a modified value for a lower air intake range and that for a higher air intake range exceed a predetermined value.
If a decision that exhaust gas is leaking is made from a modified value for a single condition of operation exceeding a prescribed value, a base air fuel ratio with an offset associated with variation in production, degradation with time and the like contributes to reduced precision for decision. The disclosed engine air fuel ratio control apparatus can use modified value for two different operation ranges to determine whether an exhaust path upstream of the catalyst has exhaust gas leak. If the base air fuel ratio has an offset, the apparatus can determine exhaust gas leak with precision.
However, the apparatus disclosed in Japanese Patent Laying-Open No. 10-47130, comparing a learning feedback value and a set feedback value to detect abnormality, provides poor precision for detection and also consumes time for detection. This is because the feedback control employs a learning value used to eliminate effects of a variety of variations to determine abnormality and it is difficult to separate the variations and the occurrence of abnormality. As such, improved precision for determination of abnormality can hardly be achieved and to provide such an improved precision a severer condition for detection of abnormality is unavoidable. This results in limited occasion for detection of abnormality and if abnormality occurs it may not be detected.
Furthermore, the apparatus disclosed in Japanese Patent Laying-Open No. 2003-206805 employs a difference between a modified value for the lower air intake range and that for the higher air intake range to detect an exhaust gas leak introduced upstream of the catalyst. This modified value is a modified value for the basic control constant for air fuel ratio feedback control that is calculated from the downstream air fuel ratio sensor's output. In this regard, the apparatus can have a disadvantage similar to that of Japanese Patent Laying-Open No. 10-47130.